Method and circuit arrangement for the detection of ground faults on electronic trips for low-voltage circuit breakers comprising serially connected measuring amplifiers

ABSTRACT

One problem during the detection of ground faults resides in the fact that the measuring amplifiers have a great amount of tolerance regarding the amplification factor thereof, resulting in a substantial error when errors are handled by means of software. According to the disclosure, the output signals of the measuring amplifiers are summed up phase by phase in a pulse-modulated manner in a summing amplifier. The duration of the pulses generated by the trip are controlled in accordance with the amplification factor of the respective associated measuring amplifier while the output of the summing amplifier represents a ground fault monitoring signal.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE2004/001906 which has anInternational filing date of Aug. 24, 2004, which designated the UnitedStates of America and which claims priority on German Patent Applicationnumber 103 42 599.3 filed Sep. 11, 2003, the entire contents of whichare hereby incorporated herein by reference.

FIELD

The invention generally relates to a method and/or a circuitarrangement. For example, it may relate to one or the other for thedetection of ground faults on electronic trips or releases forlow-voltage power circuit breakers having upstream or serially connectedmeasuring amplifiers.

BACKGROUND

Low-voltage power circuit breakers may have other functions in additionto their main function, namely that of short-circuit current monitoringand overcurrent monitoring. These other functions include, as the mostimportant function, that of ground-fault monitoring.

Ground faults may have serious consequences in electrical systems bythem triggering fires, for example. These ground faults are thereforeeliminated using power circuit breakers, which have a correspondingground-fault detection function, with a time delay by the power circuitbreaker being tripped, or the ground faults are at least indicated usingsignals.

In the case of two-pole or three-pole power circuit breakers, for thispurpose vectorial summation is carried out of the phase currents and ofthe current in the neutral conductor, if a neutral conductor currenttransformer is provided. If a residual current is above a responsevalue, a display is triggered and the power circuit breaker is tripped,if appropriate. As an alternative to this, a ground fault can also onlybe detected by an additional current transformer in the neutralconductor or at the grounded star point.

The ground-fault detection function can be implemented, for example, bya special module which can be inserted on the power circuit breaker, asis described in DE 694 25 916 T2.

The current measured by the current transformers is usually passed on tothe tripping unit of the low-voltage power circuit breaker via measuringamplifiers. One problem with this is the fact that the measuringamplifiers have a high tolerance in terms of their gain factor. Duringthe summation which is required for the purpose of detecting groundfaults, the release can then measure a ground fault although the powersupply system is free of ground faults. The reverse case is alsoconceivable, i.e. there is a ground fault but the total current isapproximately zero.

In order to alleviate this problem, until now software-orientedcorrection has been carried out in the processor of the release. Thecurrent signals have then been summated in a software-oriented manner.However, it has been shown that this results in a considerable degree oferror since the low ground-fault currents resulting in tripping onlyapproximately correspond to two converter stages of theanalog-to-digital converters of the electronic tripping unit, with theresult that the phase currents are not reproduced precisely enough forthe purpose of ground-fault detection.

SUMMARY

At least one embodiment of the invention includes an object ofspecifying an electronic release for low-voltage power circuit breakerswhich allows for error-free detection of ground faults.

Accordingly, the output signals from the measuring amplifiers are passedon to a pulse-width-modulated summing amplifier. The three or four inputsignals are connected to the input of this summing amplifier via threeor four analog switches. Owing to the selection of the respective dutyfactors of the analog switches, correction takes place which brings thegains for the inputs of the ground-fault measurement to the same desiredfactor.

One advantage of the circuit according to at least one embodiment of theinvention is the fact that hardware-oriented compensation is dispensedwith. The inaccuracies in the software-oriented addition are no longerpresent. The gain factors of the measuring amplifiers can be determinedeasily by an equal input signal being passed on to each currenttransformer in a test, and the measurement results then being stored inthe microprocessor of the release. They can then be used directly in thedetermination of the duty factor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in more detail below byway of example with reference to the drawings, in which:

FIG. 1 shows an example of a circuit according to at least oneembodiment of the invention, and

FIG. 2 shows a second variant of a circuit according to at least oneembodiment of the invention.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

FIG. 1 shows a schematic of an electronic release on a three-phase powersupply system. On the three conductors L1, L2, L3 of the power supplysystem, the currents are measured via current transformers SW1, SW2, SW3and passed on to the measuring amplifiers V1, V2, V3, whose outputs areconnected to analog-to-digital converters A/D1, A/D2, A/D3 of amicroprocessor μP, which represents the essential functional part of therelease.

According to at least one embodiment of the invention, the outputsignals from the measuring amplifiers V1, V2, V3 are passed on toswitches S1, S2, S3, which are driven in a pulse-width-modulated mannerby the microprocessor μP. The switches S1, S2, S3 (illustrated onlyschematically here) can be implemented, for example, by switchingtransistors. If one switch S1, S2, S3 is carrying a current, the othersare open, in which case the duty factor corresponds to the gain factorof the respectively associated measuring amplifier V1, V2, V3.

The outputs of the switches S1, S2, S3 are connected to a point whichrepresents the input of a summing amplifier V4 via a series resistor R.The summing amplifier V4 is implemented by an integrating circuit, i.e.by an operational amplifier OPV1, whose output is fed back to its inputvia a capacitor C1. Since the overall measurement varies within the DCrange, the second input of the operational amplifier OPV1 is biased witha reference voltage Uref, which corresponds to the mean value for thevoltage range of the measuring amplifiers V1, V2, V3. The output signalof the operational amplifier OPV1 is then passed on to ananalog-to-digital converter A/D4 of the microprocessor μP and representsa signal for ground-fault monitoring.

The circuit shown in FIG. 2 differs from the previously describedvariant only by the fact that the output signals from the measuringamplifiers V1, V2, V3 are passed on to the switches S1, S2, S3 viaresistors R1, R2, R3, and the outputs of the switches S1, S2, S3 areconnected directly to a capacitor C2, which reproduces the total currentof the phase currents of the conductors L1, L2, L3. This signal ispassed on to the analog-to-digital converter A/D4 of the microprocessorμP via an operational amplifier OPV2.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method for low-voltage power circuit breakers connected tomeasuring amplifiers, the method comprising: summing the output signalsfrom the measuring amplifiers, in a manner which is pulse-modulated interms of phase, in a summing amplifier, the duration of the pulses beingcontrolled by a release as a function of the gain factor of therespectively associated measuring amplifier, and the output of thesumming amplifier representing a signal for ground-fault monitoring. 2.A circuit arrangement for the detection of ground faults on electronicreleases for low-voltage power circuit breakers connected to measuringamplifiers comprising: switches drivable in a pulse-width modulatedmanner by the release on the basis of the gain factors of the measuringamplifiers and connected to, in each case, one output of a measuringamplifier outputs of the switches together being connected to the inputof a summing amplifier, whose output represents a signal forground-fault monitoring.
 3. The circuit arrangement as claimed in claim2, wherein the summing amplifier is an integrating amplifier, comprisingan operational amplifier having capacitive feedback, whose second inputis biased with a reference voltage, which corresponds to the mean valuefor the voltage range of the measuring amplifiers.
 4. The circuitarrangement as claimed in claim 2, wherein the summing amplifier isimplemented by a capacitor having a downstream amplifier.
 5. A methodfor ground fault monitoring on electronic releases for a low-voltagepower circuit breakers connected to measuring amplifiers, the methodcomprising: summing the output signals from the measuring amplifiers, ina manner which is pulse-modulated in terms of phase, duration of thepulses being controlled by a release as a function of the gain factor ofthe respectively associated measuring amplifier; and outputting a summedsignal for ground-fault monitoring.
 6. An arrangement for ground faultmonitoring on electronic releases for a low-voltage power circuitbreakers connected to measuring amplifiers, the method comprising: meansfor summing the output signals from the measuring amplifiers, in amanner which is pulse-modulated in terms of phase, duration of thepulses being controlled by a release as a function of the gain factor ofthe respectively associated measuring amplifier; and means foroutputting a summed signal for ground-fault monitoring.
 7. Thearrangement as claimed in claim 6, wherein the means for summingincludes an integrating amplifier, comprising an operational amplifierhaving capacitive feedback, whose second input is biased with areference voltage, which corresponds to the mean value for the voltagerange of the measuring amplifiers.
 8. The arrangement as claimed inclaim 6, wherein the means for summing is implemented by a capacitorhaving a downstream amplifier.